Flattening a film is an absolutely necessary technique in the process of fabrication of a semiconductor device. In such a situation, there has been often used a chemical mechanical polishing apparatus. In the chemical mechanical polishing apparatus, the surface of an object to be polished such as a wafer is polished by the use of a chemical polishing material containing therein polishing particles, referred to as a slurry. In the chemical mechanical polishing apparatus of this type, in order to prevent any adverse influence by a by-product generated during polishing or a change in chemical composition of the slurry per se, the slurry is continuously supplied during the polishing.
The polishing particles contained in the slurry are present in a polishing liquid as the polishing material, and they are not always dispersed in the polishing liquid in a uniform manner. Consequently, it is difficult to maintain the distribution of the polishing particles or the size distribution of the polishing particles in a constant state all the time. If the distribution of the polishing particles or the size distribution of the polishing particles is different, a change in polishing speed with respect to the object to be polished becomes marked. As a result, the difference in distribution of the polishing particles or size distribution of the polishing particles may considerably influence machining quality such as the uniformity or flatness of polishing. In particular, as shown in FIG. 10AF and FIG. 10B, the size distribution of the polishing particles or the mixing ratio of the polishing particles to the polishing liquid is usually dependent on slurry manufacturers. Therefore, the change of a manufacturer markedly influences the quality of machining. Moreover, even in the case of one and the same manufacturer, the distribution of the polishing particles or the size distribution of the polishing particles frequently varies depending on a fabrication lot. Furthermore, even in the case of one and the same fabrication lot, the content of the polishing particles contained in the polishing liquid tends to become low in the former half of slurry supplying. Thereafter, although the slurry may be temporarily supplied in an appropriate mixing ratio, the content of the polishing particles contained in the polishing liquid becomes higher in the latter half of the slurry supplying and thereafter. Additionally, there have been recently proposed a variety of techniques so that the slurry, once used in polishing, is recovered for recycling, thereby making the above-described problems more complicated. In other words, the recovered slurry for recycling is recycled after the recovered slurry is mixed with a non-used fresh slurry to adjust the components of the slurry. Therefore, the difference in distribution of the polishing particles or size distribution of the polishing particles becomes greater, so that its adverse influence on the quality of machining cannot be negligible.